This invention relates to a matrix display device for displaying information which includes optical sensing means for enabling input of information as well, for example, by way of a light pen. More particularly, the invention is concerned with an active matrix liquid crystal display device comprising a display panel having an array of picture elements each comprising a liquid crystal display element and an associated switching means, sets of row and column address conductors connected to the picture elements and drive means for applying drive signals to the sets of conductors for driving the picture elements, the display panel further including an array of active addressed sensing elements each comprising a photo-sensitive element and a switching device.
Examples of display devices of the above kind are disclosed in U.S. Pat. No. 4,345,248 which also describes typical uses, for instance as an output/input interface for a computer system through which data can be entered by writing with a light pen.
In these known examples, the display device consists of an active matrix liquid crystal display panel of generally conventional form having a row and column array of display elements connected to sets of row and column address conductors through respective thin film transistors (TFTs). The display elements are driven in formal fashion by applying a selection (gating) signal to each row conductor at a time in turn so as to turn on the TFTs of the picture elements associated with that row whereby data signals present on the column conductors are transferred to electrodes of the respective display elements in the row to produce the required display effect. A sensing element, comprising a photodiode and a TFT, is provided for each picture element. The picture element TFTs, display element electrodes, the sense element TFTs and photoelectric elements, and the row and column address conductors are all formed on a common substrate of the display panel. The sense elements are operated in a similar fashion to the picture elements with a gating signal being applied to the TFTs of each row of sense elements in turn so as to turn those TFTs on and connect the photoelectric elements of the row of sense elements to a detection circuit which is responsive to an increase in the conductance of a photoelectric element caused by illumination from the light pen. Consequently, for an input to be detected the illumination of a sense element must coincide with the gating of the TFT of the sense element concerned. The display device is operated such that the display and sense functions are performed in separate frame periods. A basic frame interval is divided into a display frame interval, during which all rows of the picture elements are addressed with display information, followed by a light detection frame interval during which the conditions of the rows of sense elements are detected. Thus there will be some time delay between successive addressing of any one sense element. Because the presence of the light pen at a particular sense element can only be detected if it is at that sense element when the sense element is addressed, then consequently for fast pen movements only a few points of movement will be detected. Moreover, because the device is operated using alternate, and temporally separate, display and detection frame intervals, the drive circuitry for the device becomes complicated and the quality of the display output is likely to be impaired. In one described embodiment, display and detection functions can be performed at the same time by using further sets of row and column conductors dedicated to the sense elements and separate from the row and column address conductors of the display elements so that the display and sense elements can be addressed independently of each other. However, the increase in the numbers of address conductors then required complicates fabrication and can lead to problems with yields.